1. Field of the Invention
The present invention relates to an apparatus and method for producing thermal fatigue cracks in a test piece for performance demonstration of nondestructive testing.
2. Description of the Related Art
Generally, in order to assure the integrity of nuclear power plant components and the stability of nuclear power plants, the safety of the components is periodically subjected to in-service inspection using nondestructive testing in consideration of the material, shape and predicted defects of the components.
Since in-service inspection of the nuclear power plants provides important results for estimating the lifetime of the plants and evaluating the safety of the components and an access to the nuclear power plants is limited, a nondestructive testing method having verified defect detection ability and high reliability should be used. In particular, the defect detection ability of an ultrasonic testing (UT) method and an eddy current testing (ECT) method has been verified to date using a simulation test piece and a mechanical process. However, there are many examples casting doubt on the ability to detect actual defects of such methods.
Accordingly, in USA, from the year of 2000, performance demonstration has been institutionalized to verify the defect detection ability of nondestructive testing, applied to nuclear components, using a test piece that simulates actual defects that are expected to occur in the nuclear components. Further, in Korea, from the year 2003, for performance demonstration of UT and ECT, the ‘Korean Performance Demonstration System’, which is centered on regulatory organizations and the owners of the power plants, has been developed.
The test piece for nondestructive verification required in ASME Section XI, Appendix VII, corresponding to the technical level applied to nuclear power plants, should include geometrical defects, implant defects, weld solidification cracks, lack of fusion, mechanical fatigue cracking, EDM notches and holes, thermal fatigue cracking, and intergranular stress corrosion cracking (IGSCC).
Moreover, techniques for producing thermal fatigue cracks, among the above defects, in a test piece, include three methods.
That is, exemplary are a first method of producing cracks comprising mounting a test piece to an autoclave and then repeatedly applying high/low temperatures to the test piece under appropriate tensile or compressive stress, a second method of producing cracks by repeatedly applying tensile and compressive stress while maintaining an appropriate temperature, and a third method of producing cracks only through repeated temperature change without the application of a mechanical load to the test piece.
However, since the test piece resulting from such a conventional thermal fatigue crack production method has properties of thermal fatigue cracks that are very different from those of actual thermal fatigue cracks occurring during operation of nuclear power plants or processing industry equipment, the effectiveness of performance demonstration of nondestructive testing cannot be reliably guaranteed.